Material transfer processes using porous molded articles are known and are widely used in medicine, industry and agriculture. Examples include slow release or controlled release processes in which an active substance, contained in the pores of a carrier substance, is required to be slowly released to the surroundings. Other examples include transport processes such as the facilitated or coupled transport of metal ions through the walls of porous membranes, as described inter alia in German Offenlegungsschrift No. 2,910,793. In these and similar processes, the membrane pores contain a complex-forming agent so that the metal ions are transported by way of their complexes.
One disadvantage of the processes mentioned above is loss of the medium with which the pores of the molded article or membrane are charged. In controlled release processes, the loss is due to the release of active substance to the surroundings and necessitates regeneration of the molded article at certain time intervals. One possible result of this loss is that the rate of release may slow down to an undesirable extent after some time, for example when a considerable proportion of the active substance has already been released. In processes for the transport of substances such as metal ions through the pores of a membrane, in which the transport is mediated or facilitated by the medium in the pores, a considerable proportion of this medium is liable to be loss after prolonged operation of the process. This initially results in a reduction of the flow rate in the process, i.e., a reduction in the quantity of metal ions transported per unit time. The process must finally be stopped or comes to a standstill of its own accord.
These known processes have the disadvantage that the charge of the porous molded articles cannot be maintained during the transfer of material. Recharging of the porous article can only take place when the transfer of material has been stopped or interrupted. Recharging is therefore not only in many cases a cumbersome process but also invariably increases the cost of the material transfer process.